Tool holders of the kind defined above are frequently present in automatic machines for metal cutting, i.e. turning, milling, drilling and the like. A benefit with this type of holder is that it allows quick and simple tool exchanges. However, a problem in connection with previously known tool holders is the adjustment of the pressure of the hydraulic medium, usually oil, that is fed into one of the chambers of the house in order to either clamp the tool or push out the same. The adjustment of the pressure is especially tricky as regards the hydraulic medium that is fed into the outer chamber in connection with clamping of the tool. If the tool is clamped by an excessively large force, the components included in the clamping mechanism may be damaged or at least, in the long run, be worn out too quickly. Therefore, it is desirable to limited the maximum pressure of the hydraulic medium that is fed to the outer chamber.
A theoretically feasible solution of the problem mentioned above would be to provide an external pressure adjusting system in the hydraulic system of the automatic machine itself, i.e. to provide, in one way or another, a fine adjustment of the pressures to both of the hydraulic medium chambers of the tool holder. However, this is for several reasons not an attractive solution. Since the pressure adjustment in such a case would take place at a distance from the tool holder, unavoidable pressure variations in the hydraulic system as a whole (as a consequence of the fact that other functions in the machine is activated or inactivated) may lead to the consequence that the outer chamber will not safely operate with a certain maximum pressure. Further, a remote controlled pressure adjusting system would run a risk that individual operators could too easily change the working pressures in the respective chambers in an uncontrolled way that is not always desired.